The present invention relates to an electron gun for a color cathode ray tube, and, more particularly, to an electron gun for a color cathode ray tube, wherein the focusing characteristic of electron beams is improved by reducing astigmatism caused by an uneven magnetic field of a deflection yoke.
The resolution of a color cathode ray tube depends on the size of an electron beam which lands on a phosphor screen. Accordingly, in order to obtain a high resolution picture, it is important that the electron beam spot projected onto the phosphor screen is as small as possible without distortion or a halo. However, in conventional color cathode ray tubes, since the RGB electron guns are arranged in-line, and a deflection yoke is adopted which deflects magnetic field as a pincushion in the horizontal direction and as a barrel in the vertical direction, astigmatism of the electron beams emitted from the electron gun and landing on the phosphor screen, occur due to the uneven magnetic field of the deflection yoke.
When the electron beams produced from such an electron gun land on the center of the phosphor screen, the deflection magnetic field does not affect them, so astigmatism of the electron beams does not occur, thus enabling the formation of a circular electron beam spot without halo. However, when deflected toward the periphery of the phosphor screen, the electron beams diverge in the horizontal direction and are excessively focused in the vertical direction due to the deflection magnetic field. Therefore, the beam spot formed on the screen has a halo in the vertical direction around its bright core which is distorted in the horizontal direction, so that picture resolution is degraded.
One example of an electron gun for a conventional color cathode ray tube is illustrated in FIG. 1, which is devised to solve the above-described problem.
The conventional electron gun has a sequentially arranged triode consisting of cathodes 2, a control electrode 3, and a screen electrode 4, and a main lens system composed of a focus electrode 5, a dynamic focus electrode 6, and a final accelerating electrode 7. Vertically-elongated electron beam passing holes 5H are formed in focus electrode 5 to correspond to horizontally-elongated electron beam passing holes 6H in dynamic focus electrode 6. A predetermined static focus voltage Vf is supplied to focus electrode 5. An anode voltage Ve higher than focus voltage Vf is supplied to final accelerating electrode 7. A parabolic dynamic focus voltage Vfd is supplied to dynamic focus electrode 6, which is synchronized with the vertical/horizontal synchronizing signals of the deflection yoke and its negative peak is the same as focus voltage Vf.
In electron gun 1 of the conventional color cathode ray tube formed as described above, when the electron beams do not deflect, in other words, when the electron beams emitted from electron gun 1 land on the center of the phosphor screen, dynamic focus voltage Vfd whose negative peak voltage is the same as focus voltage Vf is supplied to dynamic focus electrode 6. Thus the potential of focus electrode 5 and dynamic focus electrode 6 is the same, so that a quadrupole lens is not formed between them. Therefore, the electron beams simply pass through a major lens formed between dynamic focus electrode 6 and final accelerating electrode 7, and then land on the center of the phosphor screen.
In contrast, when the electron beams emitted from cathodes 2 deflect toward the periphery of the phosphor screen due to the uneven deflection magnetic field, dynamic focus voltage Vfd synchronized with a deflection signal is applied to dynamic focus electrode 6, so that a quadrupole lens having a focusing lens in a focusing section and a diverging lens in a diverging section is formed between focus electrode 5 and dynamic focus electrode 6. Due to the vertically-elongated electron beam passing holes 5H formed in the outgoing plane of focus electrode 5 and the horizontally-elongated electron beam passing holes 6H formed in the incoming plane of dynamic focus electrode 6, the lens has a weaker focusing force and a stronger diverging force in the vertical direction relative to the horizontal direction. Therefore, the electron beams passing through the quadrupole lens having the two lenses are under the influence of a force which focuses in the horizontal direction and diverges in the vertical direction, so that the cross-sectional shape of the beams becomes vertically-elongated. When the vertically-elongated electron beam deflects toward the periphery of the phosphor screen after having passed through the major lens formed between dynamic focus electrode 6 and final accelerating electrode 7, the deflecting magnetic field of the deflection yoke compensates the distortion of the electron beam caused by the uneven deflecting magnetic field. That is, the electron beam diverges in the horizontal direction, and is focused in the vertical direction. As a result, a circular beam spot can be obtained at the periphery of the phosphor screen.
However, in electron gun 1 of the conventional color cathode ray tube, since vertically-elongated electron beam passing holes 5H are formed in the outgoing plane of focus electrode 5, and horizontally-elongated electron beam passing holes 6H are formed in the incoming plane of dynamic focus electrode 6, during assembly, it is difficult to make the vertically-elongated electron beam passing holes 5H accurately correspond to the horizontally-elongated electron beam passing holes 6H. Moreover, if vertically-elongated electron beam passing holes 5H do not exactly correspond to horizontally-elongated electron beam passing holes 6H, the quadrupole lens formed between beam passing holes 5H and 6H becomes asymmetric which abnormally distorts the electron beams passing through the lens so that a desirable electron beam spot cannot be obtained To be specific, focus electrode 5 and dynamic focus electrode 6 which form the quadrupole lens are spaced apart from each other by a predetermined interval. For this reason, since an external electric field, i.e., an electric field flowing inside the neck, possibly encroaches between focus electrode 5 and dynamic focus electrode 6, the static lens may be distorted by leakage of the electric field.